LTC4413 datasheet(18/32 Pages) AD

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LTC4421

Rev. 0

For more information www.analog.com

After calculating the power, refer to the SOA curves in the

MOSFET manufacturer’s data sheet. The SOA curves are

usually specified at 25°C and must be adjusted to account

for the highest operating ambient temperature TA as given

by Equation 12.

SOA(T

A ) = SOA(25°C) •

JMAX – TA

(

)

JMAX – 25°C

(

)

(12)

Where TJMAX is the maximum allowed junction tempera-

ture of the MOSFET. Most of the recommended MOSFETs

have TJMAX =175°C. As a result, multiply the y-axis values

of the SOA curves by 0.6 for TA = 85°C, and multiply by

0.333 for TA = 125°C.

Note that MOSFET data sheets usually show a family

of 5-6 SOA curves, with each curve separated from the

next by a factor of 10 in time (e.g., 100µs, 1ms, 10ms,

etc.). To be conservative, choose the time curve closest

to and higher than tTMR,FLT and make sure the MOSFET

can handle the power in Equation 11.

When the output is hard-shorted to ground, such that the

output voltage is below 410mV, the LTC4421 implements

a stepped foldback feature, reducing the short circuit cur-

rent, and hence the power, by a factor of two. As a result,

the LTC4421 provides more SOA margin for the MOSFET

for hard shorts than resistive shorts. See procedures out-

lined in this section and the SOA curves in the chosen

MOSFET manufacturer’s data sheet to verify suitability

for the application.

APPLICATIONS INFORMATION

Overcurrent Faults and Retry

The LTC4421 features an adjustable current limit that protects

against output short circuits and excessive load current. An

overcurrent fault occurs when the current limit circuitry has

been engaged for longer than the time set by the TMR pin.

When the output load current is less than ILIM, the LTC4421

pre-biases the TMR pin voltage to its DC TMR Parking

Voltage. When the LTC4421 is regulating the output current

to ILIM, it sources 6µA out of the TMR pin to charge the exter-

nal TMR capacitor. When the TMR pin voltages increases

by 500mV from the TMR Parking Voltage, an overcurrent

fault occurs. The FAULT open-drain output pull-down pin is

latched low, and the input is disconnected from the output.

Connect a capacitor CTMRbetween TMR and ground and use

Equation 13 to set the overcurrent fault time tTMR,FLT.

TMR,FLT = CTMR • 83 µs/nF

[

]

(13)

Note that pulsed current loads exceeding the programmed

current limit and having duty cycle > 25% will integrate

over time and cause a current limit fault.

After an overcurrent fault occurs, the subsequent func-

tionality depends on the configuration of the DISABLE,

FAULT and RETRY pins. Figure 6 shows a timing diagram

for an overcurrent fault occurring on Channel 1 where

the RETRY pin is set low, FAULT1 is pulled up to a supply

voltage with a 100k resistor and the user drives DISABLE1

with a digital signal. For simplicity, there is no input supply

connected to V2.

OUT1

I(RSENSE1)

TMR1

FAULT1

DISABLE1

tTMR,FLT

COOL DOWN TIME = (1024 • tTMR,FLT)

OVERCURRENT

FAULT

OUTPUT SHORT

TO GROUND

USER

INITIATES

COOL DOWN

COMPLETE

LTC4421 F06

RECONNECT

OUTPUT SHORT

RELEASED

Figure 6. Manual Retry after Overcurrent Fault on Channel 1. Conditions: RETRY = 0V,

User Driven DISABLE1 and Output Short Released During Cool Down Time

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